All the pressures mentioned are absolute pressures and the percentages are molar percentages.
It is known to separate such a mixture in order to produce carbon monoxide and hydrogen by a methane scrubbing process as described in Linde Reports on Science and Technology, “Progress in H2/CO Low-Temperature Separation” by Berninger, 44/1988 and in “A New Generation of Cryogenic H2/CO Separation Processes Successfully in Operation at Two Different Antwerp Sites” by Belloni, International Symposium on Gas Separation Technology, 1989.
Other documents that describe methane scrubbing processes comprise: EP-A-0928937, U.S. Pat. Nos. 4,478,621, 5,609,040 and Tieftemperaturtechnik, page 418.
The carbon monoxide derived from H2/CO cold boxes entrains with it a large fraction of the nitrogen present in the feed gas. This phenomenon is linked to the difficulty in separating the two components CO and N2, their bubble points being very close. Nevertheless, depending on the use which is made of the CO downstream of the cold box, it sometimes proves necessary to reduce its nitrogen content before exporting it.
In order to do this, recourse has conventionally been made to the installation in the cold box of a column known as a denitrogenation column, the function of which is to produce, at the bottom, carbon monoxide at the required purity. At the top of the column, a nitrogen purge containing a fraction of CO is recovered. The denitrogenation column is installed either upstream, or downstream of the CO/CH4 separation column.
The reboiling of the denitrogenation column is carried out by an injection of carbon monoxide in vapor form in the bottom of the column.
This carbon monoxide comes from several sources, one of which is the vaporization of liquid carbon monoxide at medium pressure in the exchange line. This medium-pressure carbon monoxide is therefore high-pressure carbon monoxide which has been liquefied and will thus have two uses:                % providing refrigeration in the exchange line, which makes it possible to accordingly limit the low-pressure carbon monoxide requirements; and        % covering at least one portion of the reboiling needs of the column, which makes it possible to reduce the supply of medium-pressure carbon monoxide from the compressor, that is to say a specifically compressed flow (certainly at a pressure below that of the cycle, since it is only compressed to the pressure of the denitrogenation column).        
It therefore appears advantageous to maximize the portion of medium-pressure carbon monoxide vaporized.
This flow may be limited by two phenomena:                % the exchange line, which cannot obviously vaporize an unlimited amount of medium-pressure carbon monoxide; and        % the maximum fraction of reboiling that is accepted originating from the vaporized medium-pressure carbon monoxide. Specifically, it is important to be able to vary the reboiling flow without destabilizing the exchange line and therefore without varying the flow of vaporized medium-pressure carbon monoxide. Similarly, it may prove that the exchange line, for example due to too large an installed surface area, cannot vaporize the required flow (this would make other fluids exit too cold, for example the gas feed of the CO/CH4 column), and that it is therefore necessary to reduce the vaporized medium-pressure carbon monoxide, whilst the reboiling requirement is unchanged.        
Depending on the case, the flow of vaporized medium-pressure carbon monoxide will therefore be sized by the exchange line or by the maximum admissible fraction in the reboiling of the CO/N2 column. When it is possible to vaporize more medium-pressure carbon monoxide, but when there is limitation due to the reboiling and when this leads to compressing of the medium-pressure carbon monoxide in addition, there is an energy loss (which results in an a priori smaller exchange surface area).
The present invention aims to remove this constraint which leads to a sizeable energy loss on current estimates, and also to eliminate the medium-pressure gas outlet on the compressor which compresses the carbon monoxide to the high pressure (line, filter, valves, passages in the exchangers, controls, etc.).